The present invention relates to composite membranes. In particular, this invention relates to transparent composite membranes and methods for producing the same.
Polymers such as polyurethane and polyester are commonly used in making membranes and bandages. Although such neat polymers are widely used, they tend to make thick sheets and do not possess good mechanical strength. Conventional polymer composites are comprised of fiber reinforcements with thermosetting or thermoplastic polymer matrices such as Proplast, a carbon fiber/PTFE composite. Composites with tailor made properties have also been developed for specific applications in biomedical, aerospace, chemical plants and automobile industries. In the biomedical field, transparent durable polymeric composite membranes have not been successfully developed, even though thicker materials such as Kelvar/elastomer and Spectra/elastomer fabrics have been reported. For example, Kutty, S. K. N. et. al., in J. Applied Polymer Sci. 46, 471-481 (1992) described the production of Kelvar/TPU by mixing the two polymers at 180xc2x0 C. and 60 rpm, and then air drying at 105xc2x0 C. for 2 hours. Although this material is relatively strong (exceeding 40 MPa), it is thick and completely opaque. Other flexible membranes used in biomedical applications, such as those used in skin patches, are predominantly made from neat polymers and they are normally opaque due to the presence of reinforcements with a different refractive index. Their thickness are also restricted to the fibers of reinforcement, typically in the mm range. These neat polymer membranes are also not very strong, with mechanical strength typically less than 15 MPa.
Ideally, the composite membrane should be thin, non-porous, and mechanically strong. For durability in a biological application, it is preferable that the composite membrane have a crystalline or semi-crystalline component to provide the reinforcement function, as it is known that crystalline and semi-crystalline structures are more resistant than amorphous structures to the invasion of body fluid and therefore may contribute substantially to the long term stability of the membrane when used as a biomaterial. Two types of semi-crystalline fibrous membranes have been described. The first type of membrane consists of uni-axially drawn fibers. This kind of membrane have fewer pores. The second type of membrane are made from bi-axially drawn fibers, such as described in U.S. Pat. No. 4,620,956. The high-modulus and high strength fiber and film are produced by the biaxial drawing of semi-crystalline polymers. The modulus of biaxially drawn polymers is less than half of that of its uniaxially drawn counterparts, and the fibers formed in this reinforcement material are of diameters in the nanoscale (less than 100 xcexcm possible for polyethylene film). Although this membrane has the advantage of being semi-crystalline and ultra-thin, it is porous, fragile, completely opaque and non-elastic.
It is therefore highly desirable that a membrane of polymeric fibers be modified to have characteristics suitable for biomedical and other applications, such as transparency, elasticity and non-porosity. However, it is well known that elastomers such as polyurethane cannot be combined with polyethylene to produce a new transparent composite material although they have similar refractive indices, because these two materials have extremely different solubility parameters, and are therefore completely immiscible. As a result, predictions by physical principles would suggest that a combination of the two would produce an immiscible blend and an opaque product. This is the main reason for the lack of success, since there is no method known in the art to combine two immiscible polymers to form a transparent composite membrane.
It is an object of the present invention to provide a transparent composite membrane from two immiscible polymers.
It is another object of the present invention to provide a flexible composite membrane.
The present invention provides a transparent and elastic composite membrane produced from two immiscible polymers. The first polymer is a polymeric fibrous network with, interconnecting pores. The second polymer is an elastomer with a refractive index matching the first polymer. The resultant composite membrane is a transparent interpenetrated network of polymers with mechanical strength superior to the parent polymers. A method of producing the new composite membrane involves dissolving the elastomer in a suitable solvent to form a diluted elastomeric solution, impregnating the polymeric membrane with the elastomer by adding the diluted elastomeric solution to the membrane to form a wet composite membrane, and then drying the wet composite membrane to form an interpenetrated network composite membrane.